Mirrors for various uses are known in the art. For example, see U.S. Pat. Nos. 5,923,464 and 4,309,075 (all hereby incorporated herein by reference). Mirrors are also known for use in projection televisions and other suitable applications. In the projection television context, see for example U.S. Pat. Nos. 6,275,272, 5,669,681 and 5,896,236 (all hereby incorporated herein by reference).
One type of mirror is a second or back surface mirror (most common), while another type of mirror is a first or front surface mirror (less common). Back surface mirrors typically include a glass substrate with a reflective coating on a back surface thereof (i.e., not on the front surface which is first hit by incoming light). Incoming light passes through the glass substrate before being reflected by the coating in a second surface mirror. Thus, reflected light passes through the glass substrate twice in back or second surface mirrors; once before being reflected and again after being reflected on its way to a viewer. In certain instances, passing through the glass substrate twice can create ambiguity in directional reflection and imperfect reflections may sometimes result. Mirrors such as bathroom mirrors, bedroom mirrors, and architectural mirrors are typically back or second surface mirrors so that the glass substrate can be used to protect the reflective coating provided on the rear surface thereof.
In applications where more accurate reflections are desired, front (or first) surface mirrors are often used. In front/first surface mirrors, a reflective coating is provided on the front surface of the glass substrate so that incoming light is reflected by the coating before it passes through the glass substrate (e.g., see FIG. 1). Since the light to be reflected does not have to pass through the glass substrate in first surface mirrors (in contrast to rear surface mirrors), first surface mirrors generally have higher reflectance than do rear surface mirrors, and no double reflected image. Example front surface mirrors (or first surface mirrors) are disclosed in U.S. Pat. Nos. 5,923,464 and 4,780,372 (both incorporated herein by reference).
Many first surface mirror reflective coatings include a dielectric layer(s) provided on the glass substrate over a reflective layer (e.g., Al or Ag). Unfortunately, when the coating becomes scratched or damaged in a front surface mirror, this affects reflectivity in an undesirable manner as light must pass through the scratched or damaged layer(s) twice before reaching the viewer (this is not the case in back/rear surface mirrors where the reflective layer is protected by the glass). Coatings typically used in this regard are not very durable, and are easily scratched or otherwise damaged leading to reflectivity problems. Thus, it can be seen that front/first surface mirrors are very sensitive to scratching. Other possible cosmetic problems associated with first surface mirrors include pinhole formations, corrosion, adhesion, and/or reflectivity level.
For example, prior art FIG. 1 of the instant application illustrates a first surface mirror including glass/Al/SiO2/TiO2, where the Al reflective layer is deposited directly onto the glass substrate. Such mirrors suffer from problems such as poor adhesion, pinholes, poor scratch and abrasion resistance, and other durability and cosmetic problems. These durability problems are particularly evident when float glass (soda lime silica glass) is used as the substrate.
Unfortunately, the durability of first surface mirrors as shown in FIG. 1 is problematic. First, there is poor adhesion between the metal layer (Al) and the glass substrate. Second, there is poor adhesion between the metal layer (Al) and the dielectric overcoat (SiO2/TiO2). Third, if a metal layer such as Cr is added below the Al between the Al and the glass substrate, corrosion of metal(s) tends to be caused by electrochemical reactions due to the flow of electrons among metals having different free energy if a multiple-layered metal (e.g., Cr/Al) is used to improve metal/glass adhesion. Thus, such first surface mirrors suffer from yield loss on mechanical durability tests due to the delamination of Al from the glass and/or silicon oxide. In the third situation, where a metal layer such as Cr is added below the Al between the Al and the glass substrate, delamination of the coating from the glass is improved but the product sometimes fails the salt fog test due to metal corrosion.
It will be apparent from the above that there exists a need in the art for a first/front surface mirror that is less susceptible to scratching, corrosion, pinhole formations, and/or the like.